Systemic illness triggered by microbial invasion of normally sterile parts of the body is referred to as sepsis. Sepsis remains the leading cause of morbidity and mortality in critically ill patients. Current concepts suggest that organ failure and mortality in sepsis are caused by inappropriate regulation of the immune system, which manifests as excessive inflammation in the presence of immunosuppression. Recent evidence gathered in our laboratory suggests that extracellular adenosine that accumulates during sepsis modulates the immune system by activating adenosine receptors on the surface of immune cells. The production of extracellular adenosine is the consequence of the cellular release of its precursors ATP and ADP, and their sequential cell surface phosphohydrolysis to AMP by ectonucleoside triphosphate diphosphohydrolase 1 (E-NTPDase1, CD39) and then to adenosine by ecto-5'-nucleotidase (Ecto5'Ntase, CD73). We found that both CD39 and CD73 are upregulated during sepsis and have pivotal roles in restraining inflammation and organ damage by initiating adenosine receptor signaling. Specific Aim 1 will test the precise mechanisms by which CD39 and CD73 prevent excessive inflammation and organ injury during sepsis using adoptive cell transfer techniques, the cre-lox techology, and ex vivo studies on innate immune cells. Specific Aim 2 will define the molecular basis of the upregulation of CD39 and CD73 on macrophages, which may serve to curb excessive macrophage activation by producing adenosine during sepsis. The overarching goal here is to delineate the role of the ectonucleotidase cascade consisting of CD39 and CD73 in protecting against sepsis and to determine whether this cascade can be therapeutically targeted to manage patients with sepsis.